An additionally or fully recordable optical recording disk was proposed in accordance with the compact disk (generally known as CD) standard. See Nikkei Electronics, Jan. 23, 1989, No. 465, page 107; the Functional Dye Department of the Kinki Chemical Society, Mar. 3, 1989, Osaka Science & Technology Center; and SPIE, Vol. 1078, Optical Data Storage Topical Meeting, 80, 1989.
This disk has a dye layer, a reflective Au layer, and a protective layer disposed on a transparent resin substrate in this order. That is, the reflective layer is in close contact with the dye layer. As opposed to the prior art disk of the air sandwich structure wherein an air space is provided on a dye layer of the disk in order to allow pits to be formed in the dye layer, the newly proposed disk is of the close contact type wherein the reflective layer is close to the dye layer. The close contact type configuration could meet the total disk thickness of 1.2 mm required by the CD standard.
In the medium of the close contact type wherein a reflective layer is close to a recording layer containing a dye, the recording layer should have a coefficient of extinction k of 0.02 to 0.05 and an index of refraction n of at least 2.0 at the wavelength of the recording and reproducing light and unrecorded portions of the recording layer have a reflectivity of at least 60%, especially at least 70% to meet the CD requirement.
According to the CD standard, recording and reproducing operations rely on semiconductor laser having a wavelength of 780 nm. The oscillation wavelength varies somewhat among semiconductor laser products and is generally in the range of from about 780 to 790 nm. It also varies to the order of .+-.10 nm with varying temperatures.
The preferred dyes used in the recording layers are cyanine dyes of the indolenine series. Normally, indolenine cyanine dyes have a coefficient of extinction k which is so greatly dependent on wavelength that for the dye having a desired value of k at 780 nm, k would deviate off the desired range if the wavelength varies within .+-.10 nm.
The inventors previously proposed to mix two or more indolenine cyanine dyes for the purpose of improving the wavelength dependency of k (Japanese Patent Application No. 48435/1990). We have found that a mixture of two or more dyes has a low melting point, a broad softening point, and poor reliability as evaluated by heat resistance and humidity resistance. Only limited combinations of dyes can minimize the change of k with wavelength variations.
As is well known in the art, a dye layer used as a recording layer experiences a lowering in reproduction capability since a light absorbing dye is less resistant against light and likely to deteriorate in the photon mode upon repetitive reproduction. We proposed to use an ionic combination of a dye cation and a transition metal complex anion as a singlet oxygen quencher for the purposes of preventing output lowering and improving light resistance (Japanese Patent Application Kokai=JP-A No. 159087/1985, 162691/1985, 203488/1985, and 163243/1985). In this combination, the dye cation and the quencher anion are present in a ratio of 1:1. If a recording layer is formed from a mixture of a cation type dye and a quencher, then there are present four types of ion, a dye cation, a quencher anion and their counter ions in a ratio corresponding to the mix ratio of the dye and the quencher. This suggests that the use of an ionic combination, absent the counter ions of the dye and quencher, has the advantages of less output lowering and higher light resistance than the mix system. Moreover, many ionic combinations have a high distinct melting point whereas mix systems have a low melting point and a broad softening point, that is, poor thermal stability. In this respect, the ionic combinations are effective for reducing the reproduction deterioration of the heat mode and improving shelf stability as well as moisture resistance.
Since conventional quencher anions, however, have a high coefficient of extinction k, the use of an ionic combination in a medium of the close contact type yields a recording layer having increased k and decreased reflectivity therewith, failing to provide satisfactory reproduction. In order to control k to a desired value, another light absorbing dye having low k must be additionally mixed, leading to a lowering of light resistance and thermal stability. Therefore, the prior art close contact type media could not take advantage of the ionic combinations having high light resistance.